Prostate cancer is the only malignancy diagnosed by non-lesion directed systematic biopsy. Nearly 800,000 transrectal ultrasound guided (TRUS) biopsy procedures will be performed in the United States this year resulting in the diagnosis of over 200,000 cases of prostate cancer. Despite the large number of biopsies performed, current prostate biopsy schemes miss approximately 15-25% of incident cancers. Accurate prostate imaging with reliable imaging characteristics could significantly reduce unnecessary biopsies, improve diagnostic accuracy and be used to monitor tumor size or characteristics for patients on active surveillance. Photoacoustic imaging (PAI) is an emerging optical (non-ionizing) imaging technique that can provide high optical contrast images with the high spatial and temporal resolution of ultrasound. PAI is capable of producing an in-vivo three-dimensional image with higher spatial resolution (up to 500 m at tissue penetration depth of about 5 cm) than existing optical imaging modalities using photoacoustic effect (transformation of light to ultrasound). Endogenous photoacoustic signal is primarily produced from hemoglobin, making the technique well suited for imaging vascularized tissue, such as tumors. In order to provide a clinical grade device we have combined our recently improved photoacoustic detection technology, utilizing capacitive micromachined ultrasound transducer (CMUT) arrays for highly sensitive deep tissue imaging, with a TRUS platform. The goal of this proposal is to test combined transrectal ultrasound and photoacoustic (TRUSPA) imaging for guided prostate biopsy. To achieve this, the TRUSPA device will first be tested and optimized using prostate mimicking phantoms and surgically removed whole prostates with known prostate tumors. Following device validation, we will conduct a pilot test of the TRUSPA instrument in patients undergoing prostate cancer biopsy with and without the FDA approved injectable contrast agent indocyanine green (ICG). We hypothesize that the TRUSPA imaging device, with well-defined image metrics derived from initial testing, will accurately identify potential lesions for biopsy compared to traditional ultrasound guided biopsy and that the use of ICG dye will further enhance this effect.